The purpose of the study is to determine safety and efficacy of tobramycin for inhalation (TOBI) in children with cystic fibrosis > 6 months and < 6 years of age. The primary objectives are to determine 1) if the change in Pa density in lower airway cultures between baseline and Day 28 is different in the TOBI group compared to placebo; and 2) if the safety profile differs between the TOBI group and the placebo group. The primary pathogen responsible for the progressive pulmonary disease in CF is Pseudomonas aeruginosa (Pa). Studies in infants and children with CF have demonstrated that lower airway colonization with PA occurs much earlier than previously recognized. Up to 50% of patients may be colonized with Pa by the second year of life. Phase II trials demonstrated that 300 mg TOBI administered twice daily as chronic intermittent therapy for 24 weeks improved lung function and decreased sputum Pa density, intravenous antibiotic usage and hospitalization rate. The FDA has approved TOBI for management of Pa endobronchial infection in CF patients > 6 years of age. This project is the third in a series of studies to evaluate the use of TOBI in younger CF patients. Approximately 204 patients will be enrolled and screened at eight CF centers. Initial screening for Pa will be by throat and/or sputum culture, with an anticipated 60% being positive for Pa. This group will undergo bronchoscopy to verify presence of Pa in the lower airways by bronchoalveolar lavage (BAL). It is anticipated that 80% (98 patients) will have positive BAL cultures and be randomized to TOBI or placebo.